It has been known for some time that polymethyl methacrylate (PMMA)/calcium phosphate type porous composites may be employed in a series of applications, such as the filling of bone voids or as drug delivery systems for the controlled release of pharmaceuticals. In fact, these composites display a proven biocompatibility, and at the same time they succeed in wedding the mechanical resistance characteristics inherent in the polymeric materials such as PMMA with the bio-reabsorption characteristics of bioceramic materials such as calcium phosphate.
A determining aspect of such polymer-ceramic composite materials is porosity, which can be a deciding factor both for the mechanical characteristics and for the functional characteristics of the composite materials themselves. In fact, porosity allows the composite material to host staminal cells, proteins that stimulate the colonisation of the patient's staminal cells, antibiotics, growth elements, and other bioactive substances that in general promote the processes of attachment, osteointegration and/or reabsorption of the composite material.
Further, designing the porosity is particularly important, since the pores must assume specific characteristics both in shape and size as a function of the various applications of the material. In fact, the role of porosity and the degree of interconnection between the pores has been recognised as an important parameter both for the reconstruction of bone tissue inside the implanted polymer matrix and for the release periods of any pharmaceuticals inserted in the composite material.
Generally, biopolymeric porous materials are created using foaming agents or by inserting in the polymer matrix powders of particles that can be dissolved at a later stage, as, for example, soluble salts or gelatin microspheres.
The solid particles destined to create the porosity can be introduced in the melted polymer, in the monomer or mixed with the solid prepolymer before the polymerisation or reticulation reaction. During this phase, difficulties may arise due to the possibility that a few particles can remain isolated and therefore do not contribute to the formation of porosity, or that the area of contact between two particles can be very small. In such cases, the periods for the removal of the solid increase, the diffusion of bodily fluids is inhibited and a large fraction of porosity can therefore prove useless from the point of view of cellular colonisation. Porosity created using foaming agents can also entail the same type of difficulty, with the formation of a large fraction of cells that are closed or only virtually connected through fractures in the surfaces that connect one cell to another.
With the aim of resolving these difficulties, the use of biocompatible and bioabsorbable liquids has been proposed. In particular, an especially effective method according to U.S. Pat. No. 4,373,217 is the advance treatment of the ceramic material powders with these liquids, aiming to fill the porosity, at least in part, in order to avoid it becoming filled with monomer during the initial phases of polymerisation, consequently impeding the subsequent dissolution of the ceramic material and therefore the creation of the desired porosity in the final composite. Further, the article “Use of α-tricalcium phosphate (TPC) . . . ” by D. T. Beruto, R, Botter in the Journal of Biomedical Materials Research 49, 498-505, 2000, discloses the use of distilled water to create aqueous dispersions of the bioceramic material utilised, which are subsequently mixed with the polymeric material and with liquid monomer. The use of these dispersions, beyond avoiding the difficulties explained above and guaranteeing the generation of good porosity, also prevents the bioceramic materials used, as for example calcium phosphate, from absorbing part of the liquid monomer and removing it from polymerisation with the successive risk that it be released itself in the patient's circulatory system. The liquids utilised, in fact, being miscible with the bioceramic material and non-miscible with the monomer or with the polymer used, impede the contact of the latter with the bioceramic material itself.
The techniques utilised up to now, which call for the creation of aqueous dispersions of the bioceramic material, notwithstanding the fact that they succeed in resolving the difficulties described above, are nonetheless incapable of allowing for the design and achievement of a final porosity of the predetermined composite.